In recent years, silicon carbide has been increasingly employed as a material forming a semiconductor device such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) in order to allow for higher breakdown voltage, lower loss, the use in a high-temperature environment and the like of the semiconductor device. Silicon carbide is a wide band gap semiconductor having a band gap wider than that of silicon which has been conventionally and widely used as a material forming a semiconductor device. By employing the silicon carbide as a material forming a semiconductor device, therefore, higher breakdown voltage, lower on-resistance and the like of the semiconductor device can be achieved. A semiconductor device made of silicon carbide is also advantageous in that performance degradation is small when used in a high-temperature environment as compared to a semiconductor device made of silicon.
One of known methods to achieve a higher breakdown voltage of a semiconductor device is to provide a plurality of guard rings concentrically to surround an element region, with the impurity concentration decreasing toward outer guard ring regions.
Japanese Patent Laying-Open No. 2011-44688 describes a semiconductor device having a peripheral region (terminal region) in which a plurality of p type guard rings are formed at a distance from an active region. Each guard ring is a floating guard ring formed in a surface layer portion of an epitaxial layer, and is covered with a gate insulating film and an interlayer insulating film.